The present invention relates to devices for electrically igniting fuel gas emanating from a burner of the type employed for appliances such as cooking ranges, ovens, clothes driers, water heaters, and other appliances. Electrical resistance ignitors are typically employed in burner ignition systems in either parallel or series connection with an electrically operated fuel gas valve for controlling flow of fuel to the burner. In parallel systems, current is supplied simultaneously to the fuel valve and the ignitor and a flame sensor is operative to effect closing of the fuel valve, if ignition does not occur within a predetermined time. In series systems, the ignitor is electrically series-connected with an electro-thermal actuator in the valve. The ignitor in such a system typically possesses a negative slope to the resistance versus temperature relationship; and, the current flow through the ignitor is not sufficient to effect opening of the electrical fuel valve until the ignitor has reached an ignition temperature wherein the resistance of the ignitor has dropped to a level sufficient to permit flow of sufficient current to effect valve opening.
In the aforesaid series type electrical ignition systems for fuel gas burners, it is required to carefully control the resistance properties of the ignitor and valve thermal actuator in order to prevent inadvertent opening of the valve when variations in power line voltage are experienced. Typically, electrically operated fuel gas valves employ a resistance heating device to heat a bimetal operator which effects opening of the valve poppet. Variations in the resistance properties of the valve heater element, in combination with tolerances on the resistance properties of the ignitor require careful control and manufacturing processes of both the ignitor and the gas valve to prevent opening of the gas valve under conditions which could cause discharge of fuel gas without ignition.
In the manufacture of electrical resistance ignitors for fuel gas burners, it has been found difficult to control the electrical resistance properties of the ignitor during fabrication, inasmuch as the materials employed for the ignitor are generally refractory materials, such as Silicon Carbide (SIC). The electrical properties of such materials are typically controlled during manufacturing by the addition of minor amounts of dopants or impurities, and, the control of these additions has proven very difficult in mass production.
Accordingly, it has been desired to provide an electrical resistance ignitor which may be employed for igniting fuel gas emanating from a burner, and which exhibits accurately an easily controllable electrical property, properties enabling the ignitor to be mass produced in high volume for application to domestic appliances.